A signal intelligence receiver intercepts radio signals at a high sensitivity across a large bandwidth of the radio spectrum. This ability to intercept radio signals may be compromised by noise sources that effectively reduce the physical range from which the signal intelligence receiver can pick signals of interest from their origins. Frequency-hopping, frequency-scanning wideband and ultra-wideband communications receivers cannot employ simple passive narrowband preselector filters to protect amplifiers and limiters in receiver front ends from strong interference outside the communications signal bandwidth. Close proximity to multiple transmitters reduces the effective communications range of such receivers to almost zero.
This range reduction has been shown to be due to intermodulation products in the front end of the receiver. Diodes near the receiver's antenna port used for power limiting or circuit switching act as mixers. The resulting intermodulation products affect virtually every communications channel in the receiver's range.
Thus, a continued need exists for front end filtering for wide-bandwidth receivers.
One scheme proposed in the past for enabling detection of weak signals in the presence of strong interference has been to clip the strong signals to remove a portion of the interfering signals' energy. This has shown some improvement, yet this approach leaves much of the interfering signal energy present for the radio receiver to attempt to differentiate from the desired signal.
Thus, a continued need exists for a way to remove the signal power of strong narrow-band interfering signals.
Low-powered signals are recovered in the presence of high-powered interference in close spectral proximity through frequency excision. The interfering signals have a much higher power than the desired “threat” signals. This higher power of the narrow-band interfering signals shows up in the frequency domain as a much greater magnitude than the threat signals. A threshold is set in a receiver such that any frequency with a magnitude that exceeds the threshold is excised in the frequency domain. When converted back to the time domain, the high-power interference frequencies will no longer be present, thereby leaving only the low-power noise and the desired signal.
In an embodiment, a digital comb limiter combiner comprises an antenna, an input coupler, a plurality of sub-band processing paths, and an output coupler. The antenna receives an analog signal that is sent to the input coupler. Each of the plurality of sub-band signal processing paths receives the analog signal from the coupler. In this embodiment, each sub-band processing path comprises an input bandpass filter connected to the input coupler, an analog to digital converter (A/D) connected to the input bandpass filter, a digital signal processor (DSP) connected to the A/D, a digital to analog converter (D/A) connected to the DSP, an amplifier connected to the D/A and an output bandpass filter connected to the amplifier. The output of each of the output bandpass filters is then connected to the output coupler.
The input bandpass filter selects a sub-band of the analog signal and provides this sub-band signal to the A/D. The A/D converts the analog sub-band signal to a digital sub-band signal. The DSP comprises an excision threshold and instructions to convert the digital sub-band signal from a time domain to a frequency domain, instructions to excise signal components of the digital sub-band signal at frequencies at which a digital sub-band signal amplitude exceeds the excision threshold thereby producing a processed digital sub-band signal, and instructions to convert the processed digital sub-band signal from the frequency domain to the time domain. The D/A converts the processed digital sub-band signal to a processed analog sub-band signal. The bandpass filter connected to the amplifier filters intermodulation products from the processed analog sub-band signal. The output coupler connected to the output bandpass filter of each of the plurality of sub-band signal processing paths receives the processed analog sub-band signal from each of the plurality of sub-band signal processing paths and combines the processed analog sub-band signal from each of the plurality of sub-band signal processing paths into a composite signal.
In an embodiment, the input bandpass filter is cryogenically cooled. In another embodiment, the out put bandpass filter is cryogenically cooled.
In yet another embodiment, the input bandpass filters of the plurality of sub-band signal processing paths have pass bands that are contiguous with one another across the entirety of a nominal bandwidth for the digital comb limiter combiner. In another embodiment, the input bandpass filter pass band and the output bandpass filter pass bands are substantially the same.
In still another embodiment, the output of the D/A is sent directly to output coupler and the output bandpass filter and the amplifier are not used.
In an embodiment, the excision threshold used by the DSP in a sub-band signal processing path is adjustable independent of an excision threshold used by a DSP in another sub-band signal processing path. In yet another embodiment, the DSP in a sub-band signal processing path transforms from the time domain into the frequency domain and from the frequency domain into the time domain independent of a DSP in another sub-band signal processing path. In another embodiment, the DSP in a sub-band signal processing path utilizes a sample size that is independent of a sample size used by a DSP in another sub-band signal processing path.
In an embodiment, the gain used by the amplifier in a sub-band signal processing path is adjustable independent of a gain used by a DSP in another sub-band signal processing path.
In another embodiment, each sub-band signal processing path further comprises a notch filter interposed between the input bandpass filter and the A/D. In an embodiment, the notch filter is self-tuning and the self-tuning notch filter is tuned based upon a tune word provided by a co-site transmitter. In yet another embodiment, the notch filter comprises a cryogenically cooled high temperature superconductor notch filter.